Prior to the present invention, it has been known to prepare iron ore pellets by mixing iron ore with lime or similar material and pelletizing. Such processes are illustrated and described in the following representative U.S. Pat. Nos. Mayer, 3,393,066; Mayer, 3,615,352; Franklin et al. 3,205,063; O'Conner, 3,214,263; Ban et al. 3,313,617; Mills, 3,351,459; Von Stroh, 3,377,146; Imperato, 3,382,063; Imperato, 3,437,474; Imperato, 3,617,254; and Ishimitsu et al. 3,649,248. The reader may also be interested in Ban, U.S. Pat. No. 3,333,951, which describes forming a dry blend of metal ore including a solid reducing agent such as a carbonaceous material, The dry blend, which is subsequently formed into a pellet, may also include ingredients such as limestone. This dry blend may also be used to coat partially formed pellets.
The reader may also be interested in the various pelletizing and other relevant methods described in the following U.S. Pat. Nos.: Agarwall, 2,871,115; Obst et al., 3,585,025; Price, 3,169,852; Price, 3,188,195; Hanson et al., 3,301,659; Hanson et al., 3,319,949; Ban, 3,264,092; and Anthes et al., 3,326,668.
See also Veale et al., U.S. Pat. Nos. 2,806,776 and 2,806,777 describing methods of strengthening iron ore agglomerates for use in a blast furnace, including certain coating methods leading to the formation of calcium ferrites.
A number of processes for direct reduction of iron ore have been developed in recent years. Among these processes are those in which the desired reduction reactions are carried out in a shaft furnace with a gaseous atmosphere.
The raw material or feed for the shaft furnace can be natural ore but is more often an iron ore concentrate in pelletized form. These pellet feeds can have silica contents of various amounts of which the most desirable are the low-silica variety containing less than 2.5% silica. A large majority of the pellets which could be utilized in the shaft furnaces have the undesirable characteristic of sticking or clustering during reduction at high temperature which makes them unsatisfactory for direct-reduction feed. In particular, the low-silica type pellets seem to show the highest degree of clustering.
In the current state of the art the materials which exhibit clustering characteristics are usually excluded from use in a shaft furnace as they present great difficulty in regulating the material flow through the furnace. When this happens the product is of poor quality, production is reduced and often a furnace will be completely plugged. A reduction under load test to measure the degree of clustering has shown that with low-silica pellets more than 60% of the pellets are often strongly fused together.
One means of improving the pellet flow in a shaft furnace has been to incorporate a rabble arm in the furnace to rotate and break up the clusters as they form. This method, however, is costly and cumbersome to operate without high maintenance.
The objective of this invention is to improve the flow characteristics of the pellets by altering their composition. Pellet feed produced by this method flows uniformly through a shaft furnace without clustering which obviates the need for mechanical devices. It is also quite desirable to provide a direct reduction feed which reacts favorably in all types of shaft furnaces without altering the construction.